1. Field of the Invention
The present invention relates to a surface acoustic wave resonator with improved Q, a surface acoustic wave filter with improved insertion loss, a surface acoustic wave duplexer with isolation characteristics, insertion loss and power handling capability that are improved, and a communications equipment using the surface acoustic wave filter, the surface acoustic wave duplexer, or the surface acoustic wave resonator.
These surface acoustic wave resonator, surface acoustic wave filter and surface acoustic wave duplexer are collectively referred to as “surface acoustic wave device”. Surface acoustic wave devices have been widely used, for example, in the mobile communication field as RF stage or IF stage filters of cellular phones.
2. Description of the Related Art
In recent years, light and small size surface acoustic wave devices with steep cutoff performance have been used as the component device for filters, delay lines, oscillators of electronic devices utilizing radio waves.
As one kind of the surface acoustic wave devices, a ladder-type surface acoustic wave filter has been known, which includes a plural number of one terminal-pair surface acoustic wave resonators disposed on the same piezolelectric substrate, in which the resonators are connected in series and parallel in a ladder-like manner.
Usually, a surface acoustic wave resonator comprises, as shown in FIG. 32, an IDT electrode 1 including bus bar electrodes 12a and electrode fingers 13 and reflector electrodes 2 disposed adjacent to the IDT electrode 1 on the both sides thereof in the direction in which the electrode fingers are periodically arranged, which is the main propagation direction of surface acoustic waves excited by the IDT electrode 1.
Since the ladder-type surface acoustic wave filter is a small size filter with low loss within the pass band, and high attenuation outside the pass band, it is widely used as RF stage filter for cellular phones and the like.
By using two in total of such ladder-type surface acoustic wave filters for the transmission side and the reception side, an antenna duplexer can be constructed.
A duplexer is a radio frequency component having the function to separate signals into signals of the transmission side frequency band (e.g. low frequency side frequency band) and signals of the reception side frequency band (e.g. radio frequency side frequency band).
While normally, a great electric power exceeding 1W is applied to the transmission filter of a duplexer, a part of surface acoustic waves excited by the IDT electrodes of the surface acoustic wave resonators constituting the transmission filter fails to be confined within the surface acoustic wave resonators but leaks outside the surface acoustic wave resonators and propagates on the piezoelectric substrate to be received by the IDT electrodes of the surface acoustic wave resonators constituting the receiving filter. Since the surface acoustic waves that have leaked are reconverted into electrical signals, a part of signals directed from the input terminal of the transmission filter to the antenna terminal leaks into the receiving filter, which deteriorates the S/N (signal/noise) ratio of received signals. The ratio of the electric power leaking to the receiving filter to the electric power at the input terminal of the transmission filter is called “isolation”.
Recently, as the recent demand from the communications system side for products with improved specifications is becoming stronger, duplexers having superior isolation characteristics to (smaller leak power ratio than) those of conventional ones are longed for.
In addition, for further improved functionality of communications equipments, the number of components constituting the communications equipments has been increasing. For this reason, there has been a constant demand for miniaturization of the components.
Now, what causes surface acoustic waves to leak outside the surface acoustic wave resonator is described.
Surface acoustic waves excited by the IDT electrode generally propagate in a direction perpendicular to the longitudinal direction of the electrode fingers. This direction is referred to as the “main propagation direction”. If the electrode fingers are so ideally formed as to have infinite lengths in the longitudinal direction, surface acoustic waves that propagate only in the main propagation direction are excited. However, since the real devices have finite sizes, surface acoustic waves that are actually excited are bound to include a component deviating from the main propagation direction.
As shown in FIG. 32, since the conventional surface acoustic wave resonator is formed such that the longitudinal lengths of the reflector electrodes 2 adjacent to the both ends of the IDT electrode 1 are almost the same as the longitudinal length of the IDT electrode 1, the reflector electrodes 2 fail to efficiently reflect the components deviating from the main propagation direction. As a result, the components deviating from the main propagation direction leak outside the surface acoustic wave resonator.
Although surface acoustic waves excited by the IDT electrode 1 include a plurality of frequency components, since in the reflector electrodes 2, the intervals between the periodic electrodes (also called “grating electrodes”) are designed so that the best reflectivity is exerted for a specific frequency, surface acoustic waves with frequency components other than that are not reflected efficiently by the reflectors but leak outside the surface acoustic wave resonator.
In addition, when the propagation mode present in the main propagation path within the surface acoustic wave resonator and the propagation mode present in the bas bar electrodes 12a can be coupled, surface acoustic waves that are excited at intersection areas of the electrode fingers leak toward the bus bar electrodes 12a. Since the end portions of the bus bar electrodes 12a are not provided with a structure to confine surface acoustic waves, surface acoustic waves leak from the end portions of the bus bar electrodes 12a to the outside of the surface acoustic wave resonator. (e.g. refer to J. V. Knuuttila, J. Koskela, P. T. Tikka, and M. M. Salomaa, 1999 IEEE Ultrasonic Sympo., p83)
Thus, surface acoustic waves that leaked outside the surface acoustic wave resonator cause the foregoing problem. Although such a leak in duplexers was not regarded as a problem before, due to the recent strict demand for improved characteristics, it is becoming recognized as a problem.